1. Field of the Invention
The present invention relates to a wastewater treatment method, in which withdrawal of an excess sludge is reduced.
2. Description of the Prior Art
Hitherto, according to the wastewater treatment method relying on the activated sludge process, the wastewater treatment system can be steadily operated under a condition, in which the BOD volume load is within the range of about 0.3 to 0.8 kg/cm3*day (“*” represents multiplication), when wastewater is brought into contact with an activated sludge under an aerobic within an aeration tank and is then sedimented within a sedimentation tank while the sludge is in part returned to the aeration tank and is in part withdrawn as an excess sludge. (See, for example, the Non-patent Document 1 below.) On the other hand, a carrier capable of holding microorganisms in a high density have now been developed and, by using the carrier, high BOD volume load of 2 to 5 kg/m3.day can be imposed enough to allow the aeration tank to be compactized. (See, for example, the Non-patent Document 2 below)
With the conventional activated sludge process, the system must be operated with the BOD volume load of about 0.3 to 0.8 kg/cm3*day, requiring the use of a large aeration tank. In the system operation according to the activated sludge process under such a high BOD volume load, not only does the treatment become insufficient, but the settleability decreases, accompanied by difficulty in separating the sludge in the sedimentation tank in the subsequent stage to such an extent as to result in difficulty in operating the system steadily. Also with the conventional activated sludge process, about 50% of the removed BOD is said to transform into sludge, and such a so-called excess sludge is required to be finally disposed in such a way as to be reclaimed in lands or incinerated after having been withdrawn out of the system and subsequently dehydrated. It is theoretically possible to configure a system that generates no excess sludge by establishing a totally oxidized condition, in which the rate of growth of the sludge and the rate of self-oxidization of the sludge are counterbalanced with each other, instead of withdrawing the sludge. However, establishment of the totally oxidized condition within the activated sludge tank results in considerable increase of MLSS within the aeration tank, raising a problem in that a considerably large activated sludge tank must be provided for. In such case, there is another problem in that the sludge would be so refined as to make it difficult to separate the sludge by spontaneous sedimentation.
In view of the incapability of any treated water being drained unless precipitation of the sludge is achieved, the system to improve the sludge settleability is suggested, in which a load is applied to the activated sludge so that the BOD sludge load within the activated sludge tank may fall within the range of 0.08 to 0.2 kg-BOD/kg-ss*day. However, this method, in which the load is applied, has been found difficult in reducing the amount of the excess sludge withdrawn. (See, for example, the Patent Document 1 below.)
As a method of removing nitrogen contained in wastewater being treated, the three-step activated sludge method, the Wuhmann method, and the Barnad method have long been well known. Any one of those known methods includes a step of contacting wastewater containing nitrogen with nitrifying bacteria under the aerobic condition within a nitrifying tank to oxidize ammonium-nitrogen to nitrite-nitrogen and nitrate-nitrogen, and a step of contacting wastewater with denitrifying bacteria under the anaerobic condition within the denitrifying tank to reduce the nitrite-nitrogen and nitrate-nitrogen to a nitrogen gas. (See, for example, the Non-patent Document 3 below.)
With the conventional wastewater treatment method utilizing the activated sludge for treating wastewater containing nitrogen, an excess sludge tends to be generated during steps of removing the BOD and nitrogen and, therefore, it is required to perform a final disposal such as, for example, reclamation or incineration after the excess sludge has been withdrawn out of the system and then dehydrated. It is theoretically possible to configure a system that generates no excess sludge by establishing a totally oxidized condition which the rate of growth of the sludge and the rate of self -oxidization of the sludge are counterbalanced with each other, instead of withdrawing the sludge. However, establishment of the totally oxidized condition within the activated sludge tank results in considerable increase of MLSS within the aeration tank, raising a problem in that a considerably large activated sludge tank must be provided for. In such case, there is another problem in that the sludge would be so refined as to make it difficult to separate the sludge by spontaneous sedimentation. While it is generally practiced that in the sedimentation tank a coagulant is added to sediment the sludge in the course of the standard wastewater treatment, the use has not been made of the coagulant in the total oxidization tank that is used for volume reduction of the excess sludge, because addition of the coagulant into, for example, the aeration tank will allow the sludge to sediment to such an extent that no sufficient aeration can take place within the aeration tank and because a substantial amount of sludge sedimented within the sedimentation tank is drained out of the system as an excess sludge.
The inventors of the present invention have suggested a waste treatment apparatus and a method therefor, in which the total oxidization tank and the aeration tank utilizing a microorganism-immobilized carrier are utilized in combination with separation membranes to eliminate generation of the excess sludge, but have been found involving such a problem that since the separation membrane that can be suitably used has a pore size not greater than 0.1 μm, the permeate flux is extremely low and, on the other hand, the required transmembrane apparatus tends to become bulky, resulting in increase of the cost of equipments and the running cost. (See, for example, the Patent Document 2 below.) Also, no removal of nitrogen is mentioned.
The following prior art documents are available, which appear to be pertinent to the present invention:
Non-patent Document 1:
“5-tei, Kougai-bousi no Gijutsu to Houki (Suishitsu-hen) (5th-ver. Pollution Control and Regulations)” edited by Kougai-boushi Gijutsu to Houki-hennshu Iin-kai and published from Sangyo Kankyo Kannri Kyokai, 7th-ed, Jun. 12, 2001, pp 197.
Non-patent Document 2:
“Kankyo-hozen•Haikibutsu-shori: Sougougijutsu Gaido (Environment Conservation and Waste Processing: Guide to All Technologies)”, Kogyo Chousa-kai, Feb. 12, 2002, pp 70.
Non-patent Document 3:
“Baiotekunoroji-katsuyo no Koukinou-gata Kassei Odei-hou (Highly Functional Activated Sludge Process Utilizing Biotechnology)”, Gihodo Pub., May 1, 1989, pp 150.
Patent Document 1:
JP Laid-open Patent Publication No. 2001-347284
Patent Document 2:
JP Laid-open Patent Publication No. 2001-205290